The present invention relates to a process for preparing vanadyl pyrophosphate catalyst. More particularly, the present invention relates to a process for the preparation of vanadyl pyrophosphate catalyst with improved structural characteristics for the selective oxidation of butane to maleic anhydride.
Layered vanadyl hydrogen phosphate hemihydrate, VOHPO4.0.5H2O has tremendous technological importance as the precursor to the vanadyl pyrophosphate, (VO)2P2O7 which is used as the catalyst for the commercially established selective oxidation of butane to maleic anhydride [Ref. G. Centi, F. Trifiro, J. R. Ebner, V. M. Franchetti, Chem.Rev.,1988, vol. 58, p 55].
Maleic anhydride is a valuable intermediate for the production of some commercially important fine chemicals and polymers. The largest use of maleic anhydride occurs in the production of unsaturated polyesters. In the fine chemicals industry, maleic anhydride is used as a raw material for the production of succinic anhydride. xcex3-butyrolactone, 1,4-butanediol, tetrahydrofuran, fumaric acid, malic acid and D-L tartaric acid.
Vanadyl pyrophosphate (VO)2P2O7 catalyzed process of maleic anhydride synthesis by selective oxidation of butane replaced the earlier process of gas phase oxidation of benzene on a supported Vxe2x80x94Moxe2x80x94O catalyst due to economic and environmental reasons. Under typical industrial conditions 65-70% selectivity towards maleic anhydride is achieved for butane conversion of 70-85%. Carbondioxide and trace amounts of acetic acid are the only other byproducts.
Vanadyl pyrophosphate (VO)2P2O7 is regarded to be the active phase of the VPO catalyst since it is the only bulk phase that exists in equilibrated VPO catalysts. The catalytic activity of (VO)2P2O7 is known to be sensitive to its morphological characteristics and in particular the preferential exposure of the (100) crystallographic plane of the (VO)2P2O7 crystallites is highly desirable since this plane has been established to be the most active and selective for the oxidation of butane to maleic anhydride [Ref. xe2x80x9cVanadyl Pyrophosphate Catalystsxe2x80x9d. ed. G. Centi, Catal.Today, 1993, vol. 16]. The morphology of the VO2P2O7 phase is indirectly controlled through that of the VOHPO4.0.5H2O phase, since it undergoes a topotactic transformation at xcx9c450xc2x0 C. to the (VO)2P2O7 phase with retention of the microstructure and morphological characteristics of the precursor VOHPO4.0.5H2O phase [Ref. J. W. Johnson, D. C. Johnston, A. J. Jacobson, J. F. Brody, J. Am. Chem. Soc., 1984, vol. 106, p. 8123]. During this transformation the (001) plane of VOHPO4.0.5H2O is transformed to the active (100) plane of (VO)2P2O7 with retention of the morphology/surface defect of the (001) plane of the hemihydrate phase.
In view of the well established role of the (100) plane of the (VO)2P2O7 phase in catalyzing the selective oxidation of butane to maleic anhydride, the synthesis of phases with preferential exposure of this plane would be of great significance in increasing the activity of (VO)2P2O7 catalyst. The intensity ratio of the interplaner (100) and in-plane (021) x-ray reflection of (VO)2P2O7 i.e. (I100:I021) is used to determine the preferential exposure of the surface (100) planes proposed to contain the active and selective catalytic sites for selective oxidation of butane to maleic anhydride. The conventional methods of synthesis of (VO)2P2O7 catalysts typically exhibits low intensity ratios I100:I021 ca. 0.4-2 indicating that the surface (100) planes are not preferentially exposed by these methods [Ref. V. V. Guliants, J. B. Benziger, S. Sundaresan, I. E. Wachs, J. M. Jehng, J. E. Roberts, Catal. Today, 1996, vol. 28, p. 275].
There are very few reports in literature on the preferential exposure of catalytically important (100) crystallographic plane of (VO)2P2O7 phase.
Reference is made to a procedure of one pot synthesis of VOHPO4.0.5H2O with high growth of the (001) plane and its subsequent transformation to a (VO)2P2O7 phase with preferentially exposed (100) plane [N. Mizuno, H. Hatayama, M. Misono, Chem. Mater., 1997, vol. 9, no. 12, p. 2697]. The drawback of this procedure is the requirement of a costly surfactant and hydrothermal condition during the crystallization of the precursor VOHPO4.0.5H2O phase.
Reference is made to colloidal templated synthesis of (VO)2P2O7 phase [M. A. Carreon, V. V. Guliants, Chem.Commun., 2001, p 1438] with I100:I021=2.48 suggesting preferential exposure of surface (100) planes. However, this method comprises a complex procedure using mono dispersed polystyrene spheres as templating agents for formation of VO)2P2O7 with selectively exposed catalytically important (100) plane.
The main object of the invention is to provide a process for the preparation of vanadyl pyrophosphate catalysts with improved structural characteristics for the selective oxidation of butane to maleic anhydride.
Another object of the invention is to provide a simple method for the selective exposure of catalytically active (100) plane of the (VO)2P2O7 catalyst used for selective oxidation of butane to maleic anhydride.
Yet another object of the invention is to provide a process for the preparation of vanadyl pyrophosphate catalyst which exhibits high selective exposure of the catalytically active (100) plane in the (VO)2P2O7 catalyst in comparison to catalyst prepared by conventional methods.
It is another object of the invention to provide a process for the preparation of vanadyl pyrophosphate which does not require hydrothermal reaction conditions for the preparation of (VO)2P2O7 catalyst with selective exposure of the catalytically important (100) plane.
It is further object of the invention to provide a process for the preparation of vanadyl pyrophosphate which does not require costly surfactant and mono dispersed colloidal templating agents during the preparation of (VO)2P2O7 catalyst with selectively exposed (100) plane.
Accordingly the present invention provides a process for the preparation of vanadyl pyrophosphate catalyst with improved structural characteristics and useful for the selective oxidation of butane to maleic anhydride, which comprises:
i) reducing V2O5 solid by an aqueous solution of NH2OH.HCl in the presence of H3PO4 to obtain a blue solution,
ii) evaporating the solution to obtain a pasty mass,
iii) aging the pasty mass to obtain a blue solid,
iv) washing the solid with boiling water to remove the water soluble phases,
v) drying the solid to form VOHPO4.0.5H2O phase,
vi) grinding the dry VOHPO4.0.5H2O phase to a fine powder.
vii) dispersing the VOHPO4.0.5H2O powder into a mixture of dimethyl formamide (DMMF) and water (H2O) and stirring the resulting slurry,
viii) recovering the dispersed VOHPO4.0.5H2O powder from the slurry and washing with hot water,
ix) drying the powder to obtain a VOHPO4.0.5H2O solid with selectively enhanced (001) plane,
x) calcining the VOHPO4.0.5H2O solid with selectively enhanced (001) plane to obtain the desired (VO)2P2O7 phase with selectively exposed (100) plane.
In one embodiment of the invention. V2O5 solid is reduced in step (i) at a temperature in the range of 70-100xc2x0 C. while maintaining a P:V ratio in the reaction mixture in the range of 1:0.8 to 1:1.4.
In another embodiment of the invention, the pasty mass obtained in step (ii) is aged in step (iii) by heating to a temperature in the range of 100-150xc2x0 C. in air atmosphere for a period in the range of 12-36 hours.
In yet another embodiment of the invention, the washed solid obtained in step (iv) is dried in step (v) at a temperature in the range of 100-150xc2x0 C. for 6-16 hours in air atmosphere to form VOHPO4.0.5H2O phase.
In yet another embodiment of the invention, the dry VOHPO4.0.5H2O powder obtained in step (vi) is dispersed in step (vii) in a mixture of dimethyl formamide (DMF) and water (H2O) wherein the DMF to H2O ratio is in the range of 20:1-1:20 (v/v) and the resulting to slurry is stirred for 1-6 hours at a temperature in the range of 50-100xc2x0 C.,
In another embodiment of the invention, the dispersed VOHPO4.0.5H2O powder is recovered from the slurry of step (vii) by filtration.
In yet another embodiment of the invention, the VOHPO4.0.5H2O solid with selectively enhanced (001) plane obtained at the end of step (ix) is calcined in step (x) at a temperature in the range of 400-500xc2x0 C. under flowing nitrogen, at flow rate of 6-10 liter/hour for 1-6 hours to obtain desired (VO)2P2O7 phase with selectively exposed (100) plane.
In another embodiment of the invention the ratio of DMF to H2O used in the mixture of DMF and H2O is preferably in the range of 2:1-1:2 (v/v).
In another embodiment of the invention, the temperature during stirring of VOHPO4.0.5H2O powder in DMF-H2O mixture is preferably in the range of 75-90xc2x0 C.
In another embodiment of the invention, the VOHPO4.0.5H2O solid obtained shows selective exposure of the (001) plane.
In another embodiment of the invention, selective exposure of the (001) plane in VOHPO4.0.5H2O can be varied b) changing the duration of DMF-H2O treatment.
In another embodiment of the invention, the temperature used in calcination of VOHPO4.0.5H2O solid is in the range of 450-470xc2x0 C. with a gradual increase of temperature at a rate of 1-2xc2x0 C./minute under flowing nitrogen.
In another embodiment of the invention, the (VO)2P2O7 obtained shows enhanced exposure of the (100) plane and is catalytically active for the selective oxidation of butane to maleic anhydride.